Thermal considerations

The maximum junction temperature allowed for most Infineon automotive linear voltage regulators is 150°C. The thermal shutdown protection can prevent the device from direct damage caused by an excessively high junction temperature. Moreover, exceeding the specified maximum junction temperature reduces the lifetime of the device. A proper design must ensure that the linear regulator is always working beneath the allowed maximum junction temperature as specified in the datasheet of the device.

Thermal resistance

Thermal resistance is the temperature difference across a structure in the presence of a unit of power dissipation. It reflects to the capacity of the package to conduct heat outside the device. It is the key parameter to be considered in the thermal design. The most useful thermal resistance for thermal calculation is the junction-to-ambient thermal resistance R thJA. In most datasheets, junction-to-ambient thermal resistance R thJA is specified in accordance with JEDEC JESD51 standards defining PCB types and heat sink area.

Cross section JEDEC 1s0p board

Cross section JEDEC 2s2p board

Thermal calculation

Knowing the input voltage, the output voltage and the load profile of the application, the total power dissipation can be calculated:

P D = (V IN – V OUT) × I OUT + V IN × I q

The maximum thermal resistance R thJA can then be calculated:

R thJA,max = (T j,max– T a)/P D

Based on the above calculation the proper PCB type and the necessary heat sink area can be selected with reference to the thermal resistance table in the regulator’s datasheet. Below is an example of the thermal consideration for an application with TLE42754G.

As a result, the PCB design must ensure a thermal resistance R thJA lower than 28.76K/W. Referring to the thermal resistance table of the TLE42754G, only a FR4 2s2p board could be used.

Transient thermal resistance

Thermal resistance constant R thJA reflects the steady-state condition of the power dissipation. In other words, the amount of heat generated in the junction of the device equals the heat conducted away. In some applications, the worst case conditions for power dissipation occur during the transient state. The duration in transient could be far shorter than steady-state.

Thermal impedance curves characterize delta temperature rise (between junction and ambient) versus power dissipation as a function of time. In this case, the junction temperature will be a function of time: T j(t) = Z thJA(t) × P D(t) + T a

Thermal impedance curve of TLE42754 in PG-TO263 package

Tips & tricks

Calculation example in transient based on TLE42754G. The following load current profile is applied.

Application conditions:

Load current:

V IN

= 13.5V

I Q1

= 400mA

V OUT

= 5V

I Q2

= 250mA

T a

= 85°C

I Q,steady

= 100mA

PCB:

JEDEC 2s2p

t 1

= 10ms

t 2

= 10s

Determination of junction temperature T j:

P 1

= (V I – V Q) × I Q1 + V I × I q1

= (13.5V – 5V) × 400mA + 13.5V × 25mA

= 3.74W

T j,t1

= T a + P 1 × R thJA,10ms

= 85°C + 3.5K/W × 3.74W = 85°C + 13.1°C

= 98.1°C < 150°C

P 2

= (V I – V Q) × I Q2 + V I × I q2

= (13.5V – 5V) × 250mA + 13.5V × 10mA

= 2.26W

T j,t2

= T a + P 2 × R thJA,10s

= 85°C + 10.5K/W × 2.26W

= 108.7°C < 150°C

P steady

= (V I – V Q) × I Q,steady + V I × I q,steady

= (13.5V – 5V) × 100mA + 13.5V × 1.5mA

= 0.87W

T j,steady

= T a + P steady × R thJA

= 85°C + 22K/W × 0.87W

= 104.1°C < 150°C

The calculation result shows that the junction temperature of TLE42754G never exceeds the maximum threshold of 150°C. This is a valid thermal design.